The invention relates to a wind turbine, and in particular a method of yawing a rotor of a wind turbine.
A typical known wind turbine 1 is illustrated in FIGS. 1 and 2. It comprises a wind turbine tower 2 on which a wind turbine nacelle 3 is mounted. A wind turbine rotor 4 comprising wind turbine blades 5 is mounted on a hub 6. The hub 6 is connected to the nacelle 3 through a low speed shaft (not shown) extending from the nacelle front. The low speed shaft is connected to a gearbox that drives a high speed shaft and the high speed shaft drives a generator (not shown). In large wind turbines suitable for use in large scale electricity generation on a wind farm, for example, the diameter of the rotor can be as large as 100 meters or more and the mass of the nacelle, blades and hub on top of the tower is considerable and can be a few hundred tons or more. The rotor rotates about a rotational axis shown by the dashed line.
The example wind turbine 1 of FIG. 1 is an Active Stall (registered trade mark) wind turbine. When this type of wind turbine 1 is generating power and its generator is about to be overloaded, its blades 5 are pitched into the wind. That is to say, the angle of attack of the blades 5 is increased to induce stall or to cause a deeper stall. Pitch is the rotation of a blade 5 about its longitudinal axis as illustrated by the arrows 7 in FIG. 2. The blades 5 are typically pitched using a hydraulic system or electric motors (not shown).
The rotation of the nacelle 3 and rotor 4 about the longitudinal axis of the wind turbine tower 2 is called yaw, and this is illustrated by arrow 8 in FIG. 2. Wind turbines 1 generate power most efficiently when the rotor 4 of the wind turbine 1 is perpendicular to the wind direction or, in other words, the axis about which the rotor rotates is parallel to the wind direction, and wind turbines are rotated or yawed to face the wind. However, in upwind turbines, where the rotor 4 is upwind of the tower 2 such as the wind turbine 1 illustrated in FIGS. 1 and 2, as the wind direction is constantly changing, the nacelle has to be turned to face the rotor 4 to be perpendicular to the wind direction. To do this, the nacelle 3 is yawed by a powered actuator, such as an electric motor or motors operating at a single speed. This powered actuator does not operate continuously, and at times when the nacelle is not being yawed by the powered actuator, brakes (not shown) are typically provided to prevent the nacelle from being blown away from this optimum position.
FIG. 3 shows a typical yaw system 50 for yawing a wind turbine 1 such as that of FIGS. 1 and 2. The yaw system 50 comprises a drive mechanism 52 and a controller (not shown) for controlling the drive mechanism 52. In this example, the drive mechanism 52 comprises a pair of motors 54 (and, in particular electric motors) each with a pinion or small toothed gear 56 mechanically connected to its shaft 58. Bodies 60 of the motors 54 are mechanically connected to the nacelle 3 (shown as the dashed lines in FIG. 3). The motors 54 are spaced apart around the circumference of and the teeth 62 of the pinions 56 of the motors 54 engage with a yaw ring 64, which is a large ring with a toothed circumference (inner circumference 66, in this example) mechanically connected to the wind turbine tower 2. In use, when the motors 54 are operated by the controller so their shafts 58 rotate, the pinions 56 rotate and move around the yaw ring 64 causing the nacelle 3 and its rotor 4 to yaw. In known systems, the yaw rate is constant and typically around 0.3°/s. Yawing, particularly in large wind turbines 1, is slow because, as mentioned above, large wind turbine nacelles 3 are very heavy, typically a few hundred tons. Again, because of the high weight of the nacelles 3 of large wind turbines 1 a lot of energy is required to yaw them.
Yaw error is the angle between the plane the rotor 4 is in and the wind direction to which the rotor 4 is exposed. In other words, the yaw error is the angle between the rotational axis of the rotor and the wind direction. The nacelle points in the direction of the rotational axis of the rotor and so the yaw error is also the difference between the wind direction and the direction in which the nacelle is pointing.
Extreme changes in wind direction result in high yaw errors and the wind turbine 1 being exposed to very high loads. Indeed, being able to withstand such loads, so called blade flap extreme loads, are the loads that drive the design of wind turbines. The required strength is typically achieved at a cost of increased weight and expense of wind turbine components, such as blades, hub, shaft, tower and foundations. However, these very high loads may occur infrequently, for example, once every year and under particularly high loads, a typical wind turbine would have to be shut down.
The wind turbine arrangement of U.S. Pat. No. 4,298,313 uses an electric motor to yaw the rotor to increase the offset between the rotor axis and the wind direction as wind speed increases. Downwind turbines, such as that disclosed in U.S. Pat. No. 5,178,518, in which the turbine is downstream of the turbine tower, can yaw downwind automatically, without actuation, by wind blowing on vanes projecting from the nacelle. Brakes can be applied to reduce the yaw speed. Brakes are also used to reduce yaw speed in the wind turbine disclosed in European patent application No. EP 1890034. Rotary dampers can also be used to reduce yaw speed, such as disclosed in Japanese patent application No. JP 2007198167.
The inventor of the present application is the first to appreciate that by increasing yaw speed of a rotor of a wind turbine, in a direction to reduce yaw error towards zero or to zero and such that the rotor faces upwind, from a first speed to a faster second speed when at least one of a yaw error threshold and a rate of change in yaw error threshold is exceeded, that extreme loads can be significantly reduced. In other words, the yaw speed of rotation is increased to rapidly reduce yaw error when the yaw error and/or change in yaw error is high or above a predetermined threshold. As a result, the yaw error, which causes high loads, is reduced during extreme changes in wind direction. As such, the maximum loads that a wind turbine should withstand may be reduced and lighter and cheaper wind turbine components result. An increased moving yaw speed of rotation or rapid rotation may be achieved in many different ways. It is preferably achieved by operating electric motors which yaw the turbine rotor at a higher rotational speed than normal, which may be above the rated speed of the motor, for a short time period. Because these extreme gusts of wind are experienced so rarely, the higher rotational speed is also only rarely used and the rotor and nacelle may yaw at normal speeds most of the time (for example, more than 90% of the time).